In recent years, the application of Schiff base ligands on organometallic chemistry has received considerable attention,
because Schiff base ligands have hard donor-atom
frameworks, good stability and easy modification [1].
Moreover, some of these organometallic complexes are
known to serve as precatalysts in homogeneous catalysis
[2]. Schiff base contains a reactive imine functional group
that is prone to nucleophilic attack. It is well known that
alkali metals can promote the reductive coupling of imino
functional groups in transition metal Schiff base complexes,
and the new dimeric polyanionic ancillary ligands can be
formed [3]. However, similar coupling reaction is rare in
organolanthanide chemistry. Gambarotta et al. obtained
a dimeric samarium(III) complex arising from the reductive
C–C coupling of two imine groups of two Schiff base ligands during reduction of tetradentate Schiff base
(salen-type) samarium(III) chloride with metallic lithium
[4]. Here we reported the synthesis and characterization
of a novel trivalent samarium complex (2), in which three Schiff base ligands were trimerized to form the new ligand L by the reductive coupling reactions of imine groups. Furthermore, the coupling reaction of imine group of Schiff base ligand with
C=N bond of quinoline ring is first reported.
Reaction of anhydrous SmCl3 with 2 equiv. of the
sodium salt of the tridentate Schiff base formed in situ by
the reaction of A with NaH in THF, after workup, afforded the
samarium Schiff base chloride SmL2Cl(THF) (1) in good
isolated yield (Scheme 1) [5]. The composition of complex
1 is confirmed by elemental analysis, and there are the characteristic absorption bands of imine groups in the IR
spectrum.
Because reduction of trivalent lanthanide complexes by
alkali metals is a popular method for the synthesis ofdivalent lanthanide complexes, we intend to synthesize the
divalent samarium Schiff base complex via reduction of
complex 1 with sodium, but the attempts were unsuccessful.
Complex 1 reacted with sodium in THF smoothly, and a deep red, air-sensitive solution formed. Dark-red crystals were isolated in reasonable yield upon crystallization from DME (Scheme 1), which was identified to be a novel trivalent samarium complex (2) by elemental analysis, and X-ray crystal structure determination [6,7].
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